How does water hardness affect the corrosion rate of hot-dip galvanized steel immersed in water?

Predicting the service life of hot-dip galvanized steel in water immersion service is more difficult than predicting it in atmospheric applications because there are many factors that affect the corrosion rate of galvanized steel in water. Some of these factors include the temperature of the water, pH, agitation, depth, oxygen, nitrogen, carbon dioxide, chlorides, sulfides, magnesium, calcium, micro-organisms, tide conditions, impurities, and whether the water is hard or soft. This article will focus only on the hardness of water and how it affects the corrosion rate of hot-dip galvanized steel. When galvanized steel is exposed to the atmosphere, periodic wet and dry cycles due to natural weather phenomena cause the zinc patina to form on the galvanized coating over time. In water immersion service, alternating wet and dry cycles do not exist since the galvanized coating is always wet. This means the zinc patina does not form on galvanized steel underwater like it does in atmospheric exposure. Normally, this would be cause for concern since pure zinc is a chemically active metal and accelerated corrosion would be expected; however, protective scales can form on the galvanized coating depending on the levels of ions in the water.

The ions that form protective scales on galvanized steel in water include magnesium and calcium cations (Mg2+ and Ca2+). The higher the level of these ions, the harder the water is considered. Moderately hard and hard water provide enough magnesium and calcium cations to form protective scales on the galvanized coating, which act as a barrier and provide corrosion protection to the underlying zinc and steel. Agitation of the water can remove these scales, which is why corrosion in the splash zone is particularly aggressive on galvanized steel. A common measure of water hardness is parts per million (ppm) or milligrams per liter (mg/L), which are equivalent measurements. Waters with hardness levels below 17 ppm are considered soft, while waters with hardness levels between 60-120 ppm are considered moderately hard, and that above 120 ppm are considered hard. The general trend for soft and hard waters is soft water is much more corrosive to galvanized steel than hard water because protective scales do not form on the galvanized coating in soft water. Of course, water hardness is only one consideration when estimating corrosion of galvanized steel in water immersion service. For more information on the performance of galvanized steel in water, please email the AGA Technical Department.